atof

Defined in header `<stdlib.h>`
double atof( const char* str );

Interprets a floating-point value in a byte string pointed to by str.

Function discards any whitespace characters (as determined by isspace) until first non-whitespace character is found. Then it takes as many characters as possible to form a valid floating-point representation and converts them to a floating-point value. The valid floating-point value can be one of the following:

decimal floating-point expression. It consists of the following parts:

(optional) plus or minus sign
nonempty sequence of decimal digits optionally containing decimal-point character (as determined by the current C locale) (defines significand)
(optional) e or E followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent to base 10)

hexadecimal floating-point expression. It consists of the following parts:

(optional) plus or minus sign
0x or 0X
nonempty sequence of hexadecimal digits optionally containing a decimal-point character (as determined by the current C locale) (defines significand)
(optional) p or P followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent to base 2)

infinity expression. It consists of the following parts:

(optional) plus or minus sign
INF or INFINITY ignoring case

not-a-number expression. It consists of the following parts:

(optional) plus or minus sign
NAN or NAN(char_sequence) ignoring case of the NAN part. char_sequence can only contain digits, Latin letters, and underscores. The result is a quiet NaN floating-point value.

(since C99)

any other expression that may be accepted by the currently installed C locale

Parameters

str

-

pointer to the null-terminated byte string to be interpreted

Return value

double value corresponding to the contents of str on success. If the converted value falls out of range of the return type, the return value is undefined. If no conversion can be performed, 0.0 is returned.

Notes

The name stands for "ASCII to float".

Example

Run this code

#include <stdlib.h>
#include <stdio.h>
 
int main(void)
{
    printf("%g\n", atof("  -0.0000000123junk"));
    printf("%g\n", atof("0.012"));
    printf("%g\n", atof("15e16"));
    printf("%g\n", atof("-0x1afp-2"));
    printf("%g\n", atof("inF"));
    printf("%g\n", atof("Nan"));
    printf("%g\n", atof("1.0e+309"));   // UB: out of range of double
    printf("%g\n", atof("0.0"));
    printf("%g\n", atof("junk"));       // no conversion can be performed
}

Possible output:

-1.23e-08
0.012
1.5e+17
-107.75
inf
nan
inf
0
0

References

C11 standard (ISO/IEC 9899:2011):

7.22.1.1 The atof function (p: 341)

C99 standard (ISO/IEC 9899:1999):

7.20.1.1 The atof function (p: 307)

C89/C90 standard (ISO/IEC 9899:1990):

4.10.1.1 The atof function

Language
Headers
Type support
Program utilities
Variadic function support
Error handling
Dynamic memory management
Strings library
Algorithms
Numerics
Date and time utilities
Input/output support
Localization support
Concurrency support (C11)
Technical Specifications
Symbol index